1. Field of the Invention
This invention relates to a suspension device for automobile. More particularly, it relates to a suspension device for automobile in which a damping force of a shock absorber and spring constant of a suspension spring for the suspension device are changed over depending on vibratory motion in the vehicle body.
2. Discussion of Background
It has so far been known that, for the sake of comfortableness in vehicle riding, the damping force required for the purpose should preferably be as small as possible and the spring constant be as small as possible (i.e., soft spring mode), and that, for the sake of steering stability, the damping force required for the purpose should preferably be as large as possible and the spring constant be as large as possible (i.e., hard spring mode). For meeting such requirements, there has been proposed a change-over system of a type such that, in an ordinary cruising condition of the automobile, the damping force of the shock absorber and the spring constant of the suspension spring may be set at small values, and, depending on necessity, both damping force and spring constant may be changed over to large values, or hard spring mode.
As means for detecting whether it is necessary to perform the change-over operations, or not, consideration will now be given to a case, wherein measurement is taken on vibratory acceleration or changes in road clearance of the vehicle body with respect to the ground surface due to its vibratory motion, or changing speed of such changes in the road clearance, from which results the change-over operations are performed.
Vibrations which the vehicle tires pick up from the ground surface are determined by the vehicle cruising speed and the surface conditions of the road. Of the vibrations, the high frequency vibrating components having very small amplitude have to be absorbed by the tires and dampers, hence the suspension spring of the vehicle body should also be made soft. On the contrary, when a large force is imparted to the vehicle body owing to operations of the steering handle or the brake, the vibratory acceleration and the amplitude are large, even though the period of vibrations is long. In such case, the change-over operations should be done in such a manner that the damping force and the spring constant are set at large values so as to suppress variations in posture of the automobile.
Simple determination of whether such change-over operations be done, or not, on the basis of the vibratory acceleration (G), or magnitude of variations and the road clearane, or their changing speed alone would cause such disadvantage that, in the case of, for example, running on a rough road, both damping force and spring constant are changed over to large values (hard spring mode) owing to the vibratory acceleration (G) to be generated by irregularities on the road surface with the consequent worsening in the ride. A further disadvantage derived from such simple determination is that, when both damping force and spring constant are changed over to large values (hard spring mode), larger vibratory motion occurs in the vehicle body running on the rough road and the vibrations assume high frequency, on account of which the vehicle body is unable to restitute its required comfort.